transport-xmpp: MAXS-2 Use MemorizingTrustManager

This commit adds MemorizingTrustManager as new git submodule within
transport-xmpp, which is configured as Android library project to
transport-xmpp.

Smack's ConnectionConfiguration truststore settings where removed, since
this is all now handled by MTM. This also fixes MAXS-3, since
MemorizingTrustManager will first try the systems TLS truststore.

Within transport-xmpp, Context had to be replaced with Application,
because MTM uses Application to retrieve the directory for its
keystore.